1. Field of the Invention
The present invention provides a communication apparatus like a facsimile machine, which implements communication according to the procedures that are defined by Recommendations T.30, V.8 and V.34 of the ITU-T (International Telecommunication Union for Telecommunications).
2. Description of the Related Art
Recommendation T.30 of the ITU-T defines a procedure for transmitting text via facsimile over a public switched telephone network (PSTN), Recommendation V.8 specifies a session start procedure for data transmission over the PSTN, and Recommendation V.34 defines the specifications of a modem which operates at a data signal rate of up to 33600 bps that is used in the PSTN and a2-wire, point-to-point telephone-type private line.
With respect to a prior art disclosed in, For example, Japanese Unexamined Patent Publication JP-A-8-79483 (1996), a communication apparatus operable according to the procedure of Recommendation V.8 is permitted to receive data both from a station which is operable according to the procedure of Recommendation V.8 and a station which is operable according to the procedure of Recommendation T.30 in the following manner. When receiving a calling menu (CM) signal within a modulated answer tone (ANSam) transmission time, the device transmits a common menu (JM signal) to execute the procedure of Recommendation V.8. When the device cannot receive the CM signal within the ANSam transmission time, on the other hand, the device transmits a non-standard facilities (NSF), a called subscriber identification (CSI) and a digital identification (DIS) signals, and upon reception of the CM signal thereafter, the device transmits the JM signal and then executes the procedure of Recommendation V.8, while upon reception of a non-standard facilities set-up (NSS), a transmitting subscriber identification (TSI) and a digital command (DCS) signals instead of the CM signal, the device executes the procedure of Recommendation T.30.
In addition, Japanese Unexamined Patent Publication JP-A 8-214136(1996) discloses a technology, wherein if the JM signal from a called station cannot be received during the transmission of the CM signal from a calling station, the half-duplex transmission system is selected by switching after the end of the transmission of the CM signal in order to receive the JM signal; furthermore if the called station cannot receive the CM or CJ (CM terminator) signal during the transmission of the JM signal, the half-duplex transmission system is selected by switching after the end of the transmission of the JM signal in order to receive the CJ signal, thereby to effectively use the functions provided by Recommendation V.8.
In the case of a communication apparatus capable of carrying out communication in accordance with the procedures of Recommendations T.30, V8 and V34, while a communication line is connected and the procedures of Recommendations V8 and V34 are executed, the communication apparatus is configured so that the communication line can be disconnected in accordance with the procedures. Therefore, if the line condition is improper because of a low signal level or the like, the procedures cannot be interrupted in the middle of transmission, whereby a longer time may be taken to detect a desired signal, or communication errors may occur. In particular, the communication in accordance with the procedures of Recommendations V.8 and V34 is more susceptible to the line condition than the communication in accordance with the procedure of Recommendation T.30. For this reason, communication cost may increase, or desired data cannot be transmitted or received.